Spray guns for the electrostatic deposition of powdered coating materials generally comprise an insulating barrel through which an air-borne stream of the solid particles flows to be charged electrostatically either within a spray cap carried by the barrel or upon issuance therefrom. Various forms of charging electrodes have been proposed and almost without exception have been less efficient than necessary and have frequently been subject to the accumulation of powder particles where the electrode protrudes into the powder stream at the discharge end of the gun. If the electrodes are outside the powder stream or spray pattern the charge on each of the spray particles is low and the charging process is inefficient. If the electrode is disposed within the spray pattern there is a tendency to accumulate powder on the electrode which then blows off in globs and causes spots or other defects in the coating. This is especially true if the electrode is of a convoluted or complex shape. The present invention has as one of its features the provision of a needle electrode within the spray pattern which is constantly washed and kept clean by a surrounding air stream of such low volume that it will not disturb the spray pattern. The velocity of the shielding or washing air stream is kept high for best effectiveness, as the air shield must persist for the full length of the needle.
Electrostatic powder spray guns are frequently used to coat relatively complex parts which include not only large flat surfaces but also interior corner areas and separated elements such as ribs or bars. It is well known that the large flat surfaces will accept an electrostatically charged powder at a high rate so that the air flow through the powder pump and the resulting powder flow through the gun may be high. If an air stream is used to expand or otherwise shape the powder stream by imparting, for example, a whirling motion thereto then the volume of air in this vortex forming stream should be high when the powder carrying air flow is high. Conversely, if the electrostatic spray gun is being used to coat a portion of the work that requires only a low powder flow such as an interior corner or an isolated bar or rib then the powder flow should be at a lower volume and the volume of the vortex forming air stream may be correspondingly reduced. Thus the operator has certain choices of spraying conditions that he would like to be able to make prior to the start of a spraying operation.